This invention generally relates to solderless printed wiring board modules, and is specifically concerned with an improved system and method for electrically, mechanically and thermally connecting at least one integrated circuit die to a printed wiring board which utilizes a fanout interface integrally connected to the die.
Solderless printed wiring board modules are known in the prior art. Such modules comprise a printed wiring board (PWB) having one or more arrays of contact pads (known as "footprints" in the art), a heat conductive guide plate assembly having a plurality of apertures that are registerable with the pattern of contact pad arrays on the PWB, each aperture being matable with an electrical component, and a connecting interface formed from a piece of compliant, insulating sheet material that is receivable within one of the apertures of the guide plate assembly. The interface includes a plurality of resilient electrical connectors (called "button connectors" in the art) that are registerable with the contact pads of the PWB when the connecting interface is dropped into one of the apertures of the guide plate assembly. Each of these button connectors is formed from a small, springy cylinder of randomly compressed, thin gauge copper-beryllium wire that has been gold plated. Each of these connectors is frictionally received within a through-hole present in the sheet material of the interface.
In operation, the interface is first dropped into one of the apertures of the guide plate assembly. The shape of the outer edge of the compliant sheet material forming the interface is matable within the aperture in the guide plate assembly, and the button connectors are registerable with the contact pads of the PWB such that the lower end of each of the connectors engages one of the pads in the PWB. An electrical component is next dropped into the aperture of the guide plate. This component is typically an integrated circuit die that has been hermetically sealed within a ceramic package housing having an external array of contact pads on its bottom surface. The shape of the outer edge of the electrical component is likewise matable with the aperture in the guide plate assembly such that its contact pads become aligned with the upper ends of the button connectors held by the sheet material of the connecting interface. When the electrical component is compressed downwardly toward the PWB by a slug retainer which screws into the upper end of the aperture, each of the resilient button connectors provides a positive, spring-loaded engagement between the pads of the PWB, and the corresponding external pads on the electrical component. Such a PWB module is disclosed and claimed in U.S. Pat. No. 4,933,808 invented by Messrs. Bourdelaise, David Harris and Roald Horton, and assigned to the Westinghouse Electric Corporation.
Such solderless PWB modules provide a great many advantages over conventional, soldered printed wiring board modules. The elimination of the soldering operation between the pads of the electrical components and the printed wiring board eliminates up to 67% of the required assembly time, and further greatly reduces the possibility of component damage due to overheating. Such solderless PWB modules may provide a higher degree of reliable interconnection between the electrical components and the printed wiring boards, as there is no possibility of defective connections occurring as a result of solder splattering or defective solder joints. Because of the need for inspecting finished solder joints, soldering encouraged perimeter-type connections to be made between the electrical component and the printed wiring board, where the finished connections could be seen. In instances where the electrical component had a large density of leads projecting out around its perimeter, the margin for error became very small for the proper soldering of each of these leads to its respective contact pad on the PWB as the distance between the leads became very small. By contrast, the advent of solderless PWB modules which utilizes a sheet-like connecting interface made it much more practical to spread these connections out in an area array pattern, where the connections are uniformly spaced apart from one another throughout the entire area under the component, as opposed to locating these connections only around the perimeter, thereby reducing the probability of an assembly error. Finally, such solderless PWB modules have much more effective heat sink properties than their prior art counterparts, and are far easier to test, repair and to replace components on.
Unfortunately, the applicants have observed that these last advantages of improved heat sink properties, ease of testing, and replacement of components are presently limited by the structure of present-day integrated circuits. Specifically, such circuits generally comprise a die formed from a silicon substrate which is hermetically sealed within a package housing. Presently, two configurations of such hermetically packaged integrated circuit dies are available, each of which has its advantages and disadvantages. These two types of packages integrated circuit dies are illustrated in cross-section on FIGS. 1 and 2, and are referred to in the art as "cavity down" and "cavity up" type integrated circuit packages, respectively. Each of these two types of packages 1 generally includes an integrated circuit die 2 formed from a rectangular substrate 3 of silicon that is received within a cavity 5 within a rectangular package housing 7. The rectangular substrate 3 forming the die 2 includes a metallized side 9 where all of the components of the integrated circuit are formed, as well as a heat transfer side 11 that is used to conduct away the heat generated by the circuit. Disposed around the edge of the metallized side 9 are a plurality of perimeter bond pads 13. These bond pads 13 are connected to an internal array 15 of contact pads disposed on a step-like structure that circumscribes the cavity 5 within the package housing 7. A plurality of fragile, hair-fine bond wires 17 interconnect the perimeter bond pads 13 with the internal array 15 of contact pads. The internal array 15 of contact pads is in turn interconnected to an external array 19a, 19b of contact pads disposed on the side of the package housing 7 that ultimately engages the "footprint" of contact pads on the PWB. In both of these packages 1, the cavity is hermetically sealed off by means of a metallic lid 21 held thereon by a bonding strip 23.
In the "cavity down" configuration illustrated in FIG. 1, the heat transfer side 11 of the rectangular substrate 3 of the integrated circuit die 2 is advantageously directly bonded to the ceiling of the cavity 5, which in turn affords a relatively short thermal path for the heat generated by the circuit. However, since the external array of contact pads 19a must be placed around the opening in the package housing 7 leading into the cavity 5, these contact pads 19a must be arranged in perimeter fashion. Such a perimeter arrangement of the contact pads 19a not only disadvantageously increases the size of the footprint area necessary for the corresponding contact pads on the PWB; it further disadvantageously crowds the contact pads 19a, thus lowering the margin of manufacturing error which can be tolerated, and enhancing the chances for an inadvertent short circuiting to take place between adjacent pads 19a. By contrast, the "cavity up" circuit package 1 illustrated in FIG. 2 can advantageously arrange its external array of contact pads 19b into an area-type configuration, as all of the cavity floor is available for this purpose. However, it does so at the expense of diminished thermal transfer ability, as it is difficult, if not impossible not to leave at least some kind of air gap between the upper surface of the substrate 3 of the die 2, and the metallic lid 21. Such spacing is necessary to insure that no short circuiting or external shock or stress will occur between or on the bond wire 17. Other problems associated with the package housing 7 used in the prior art include the extra size and weight that such housings impose on the integrated circuit dies 2, as well as the difficulty of repairing or replacing such a die 2 within such a housing 7, which requires the removal of the bonded lid 21 and the severing and replacement of the hair-fine bond wire 17. This last problem is a particularly expensive one in a case where several dies are contained within a single package housing to form a circuit sub-assembly, where the malfunction of a single die often causes the entire sub-assembly to be scrapped.
Clearly, what is needed is a system and method for mechanically and electrically connecting an integrated circuit die to a printed wiring board through an area-type arrangement of contact pads, but which has all of the desirable thermal dissipation characteristics of a cavity down-type integrated circuit package. Ideally, the size and weight of the package housing would be little bigger than the integrated circuit die itself, to provide the smallest and most lightweight solderless PWB module possible. Finally, in the case where several integrated circuit dies are enclosed within a single package housing, it would be desirable if some sort of means were provided for quickly and easily gaining access to and replacing one or more of the dies so that repairs might be easily made on such subassemblies.